Efforts were made to improve the growth of mouse megakaryocytes in cultures. Now good granular megakaryocytes are regularly observed on day 0. A few cells showing beautiful demarcation membrane formation are observed on day 2 to 3. Marked increases in the number of cells, mostly the megakaryoblasts, appear on day 3, reaching a peak on day 5 to 6. Apparently 500 megakaryocytes per 2 x 10 to the 5th power nucleated marrow cells are obtained at this stage. Well developed promegakaryocytes are routinely observed on day 4 to 5, and quite a few granular megakaryocytes are also observed at this stage. Unfortunately, the latest mature form, cells showing demarcation membrane systems, is observed only infrequently. Studies to improve cell maturation are in progress.